The present invention relates to process transmitters. In particular, the present invention is a process transmitter featuring digital compensation with improved dynamic performance.
Process transmitters are used to monitor industrial process parameters (or process variables) such as differential pressure, gage pressure, absolute pressure, fluid flow, liquid level, temperature, pH, etc. Modern high performance process transmitters use digital signal compensation to achieve low total error. Uncompensated process signals are digitized using an analog-to-digital (A/D) converter and passed to a host microcontroller for digital compensation. Ambient temperature information is also digitized and passed to the host controller for the purpose of temperature compensation. Factory characterization produces correction coefficients that allow the device output to be very linear and temperature compensated, which provides a very low total error for static input conditions.
One consequence of this digital compensation scheme has been a sacrifice in dynamic performance. The A/D process in addition to the digital compensation process adds a significant amount of dead time to the transmitter. Typical process transmitters with digital compensation may have dead times ranging from 100 mS to 500 mS. Dead time may be problematic for applications which require fast control loops such as pulp/paper head box pressure control, or emergency shutdown applications.
Process transmitters dating back to the 1970's were all analog in nature. Lacking A/D converters and digital processors, these devices had virtually no dead time, and responded very rapidly to dynamic input signals. Unfortunately, their total performance was poor by today's standards.
There is a need for process transmitters that offer the static performance of a digitally compensated device, and the dynamic performance of an all analog device that is free of dead time.